This prospective, randomized, open-label, phase II clinical trial compares two sequential intravesical chemotherapy regimens in patients with bacillus Calmette-Guérin (BCG)-unresponsive high-grade non-muscle-invasive bladder cancer (NMIBC) following complete transurethral resection of bladder tumor (TURBT). Participants are randomized in a 1:1 ratio to receive sequential intravesical gemcitabine plus docetaxel or sequential intravesical gemcitabine plus mitomycin C. The primary objective is to compare 12-month event-free survival between treatment groups. Secondary objectives include evaluation of cystectomy-free survival, progression-free survival, treatment safety, tolerability, and patient-reported quality of life.